This invention relates to distance measuring equipment for use on aircraft and more particularly to means for starting and stopping the range clock of such equipment in response to an interrogation and subsequent response thereto so as to compensate for certain errors or uncertainties.
In a round trip ranging means such as the DME an interrogation is made by an interrogating station or interrogator at a time defined as t.sub.O. A remote station responds to the interrogation and transmits a response. The time interval between t.sub.O and the arrival of the response at the interrogator is a measure of the distance or range from the interrogator to the remote station. Various elements of the interrogator or uncertainties associated with the interrogator can cause range errors. For example, range error can arise from filter time delay, amplifier propagation delay, detection, thresholding quantization, modulation and transmitter characteristics, etc. These range errors are normally present in or introduced by the receiver section of the normally airborne interrogator. One object of the present invention is to provide a highly accurate range measurement by eliminating, compensating for or calibrating to attenuate various of the range errors.
In conventional DME interrogating equipment the transmitter section of the equipment operates at a frequency which is different than the frequency at which the receiver operates. This eliminates interaction between the transmitter and the receiver and allows means for distinguishing between interrogations and responses thereto. The range clock is generally the last element at the back end of the receiver. Upon transmission of an interrogation by the interrogator a signal is applied directly from the transmitter section to start the clock. Then, of course, upon receipt of a response, the response is processed and decoded by the receiver section and applied to stop the range clock.
In addition, conventional interrogating equipment, of course, includes threshold circuits at the input of the range clock, with these thresholds to be exceeded before the range clock can be activated or stopped as the case may be. It is also well known to those skilled in the art that a response to a DME interrogation is subject to various causes of distortion, such as multipath distortion. This distortion is particularly severe and of concern at the leading edge of the response, and particularly at the back end of the leading edge. It is thus advantageous that the threshold be set as low as practical so as to respond to that portion of the leading edge which is least distorted. However, the slope of the leading edge of a received signal will vary as the signal strength varies. Specifically, a low strength received signal will be detected much later by a low level threshold than a high strength received signal would otherwise have been detected. The strength of received signals is affected by various factors such as range between stations and weather conditions.